Some species play a special role in their community structure. Dominant species are those that dominate the community, either in abundance, as in the number of them, or biomass, how much, you know, organic matter they take up in the community. Now an example of dominant species are these mangroves, which tend to dominate communities that are in brackish swamp waters. Now, keystone species are species that have a significant effect on their community, but they're not the dominant species. And when you remove a keystone species, you dramatically change that community structure. A great example of this are these starfish, which play a super important role as predators in tide pools. And without them, sea urchin populations go just crazy. Now, lastly, I want to talk about ecosystem engineers. The most obvious example of which is the North American beaver. These organisms, these ecosystem engineers, significantly modify or maintain their habitat. And beavers, as we know, create dams. They literally engineer their habitat. Though some people don't like using the word 'engineer' because they think it implies too much critical thinking in the process. Whereas, you know, a lot of these behaviors are going to be more instinctual.

Now, communities will change when they experience a disturbance. A disturbance is a temporary change in the environment, and it's going to result in a change in the ecosystem. This includes stuff like wildfires, or storms, floods, even disease outbreaks. And they're going to change communities by removing organisms or altering resource availability. And sometimes there's actually a recurring disturbance pattern that a community will experience, and this is known as a disturbance regime. Now the non-equilibrium model says that communities are constantly changing after disturbances. That is, they don't, you know, just stay where they are; they develop. And sort of in conjunction with that, the intermediate disturbance hypothesis says that moderate levels of disturbance actually lead to greater species diversity. So, in a sense, moderate levels of disturbance are better for the community.

Now here we can see a great example of the aftermath of a disturbance. Notice how all of this is green and all of this is charred. This is the remains of a wildfire. This graph right here represents the intermediate disturbance hypothesis, which shows that species diversity on the y-axis is going to be highest at an intermediate level of disturbance. Here we go. Intermediate level of disturbance. See? Right in the middle. Too much disturbance, species diversity plummets. Too little disturbance, species diversity isn't as high as it could be. And with that, let's go ahead and flip the page.